Gas discharge lamps are commonly known, so an elaborate discussion of the design of a gas discharge lamp is not needed here. Suffice it to say that a gas discharge lamp comprises two electrodes located in a closed vessel filled with an ionizable gas or vapor. The vessel is typically quartz or a ceramic, for instance polycrystalline alumina (PCA). The electrodes are arranged at a certain distance from each other, and during operation an electric arc is maintained between those electrodes.
A gas discharge lamp may be powered by an electronic driver. Electronic drivers are commonly known, so an elaborate discussion of the design of electronic drivers is not needed here. The driver produces lamp current; although in principle direct current operation would be possible, the lamp current is typically a commutating current, i.e. a current with substantially constant magnitude but alternating direction; this is also indicated as square wave current. The commutation frequency may for instance be in the order of about 100 Hz.
One particular application for HID lamps is as light source in a projection system. In this field, there is a desire to make smaller panels (to reduce costs) and to increase the lamp power (to increase the light output). Further, it is desirable that the lamps have a long lifetime, and that the light output (lumen) of the lamp is substantially constant over the lifetime.
For smaller panels, the main parameter that influences the light output (lumen) of the lamp is the arc length, so the electrode distance needs to be as constant as possible over the lifetime. Further, the light output should be stable, thus the arc should be free from flatter, which puts requirements on the quality of the electrode surfaces.